Currently, petroleum is used as a main energy source. However, serious problems occur due to emission of air pollutants and global warming caused by a so-called greenhouse effect. To regulate emission of carbon dioxide, a main cause of global warming, The Kyoto Protocol was adopted in 1997 and came into effect in Feb. 16, 2005. Korea has been included in countries under regulation since 2013. Therefore, it is necessary to diversify energy supply sources. There is an imminent need to develop renewable energy sources such as solar energy, wind power and water power that substitute for petroleum energy.
Solar cells are classified, depending on their constituents, into those including inorganic materials such as silicon compound semiconductors and sensitive dye molecules. When the particle size is excessively small (<several nanometers), dye adsorption increases. On the contrary, in this case, the number of surface states increases to provide recombination sites undesirably. Therefore, it is a key solution in dye-sensitized solar cells to control the particle size, shape, crystallinity, microstructure and surface characteristics of oxide.
The oxides that have been studied to date include TiO2, SnO2, ZnO, Nb2O5, or the like. Among such oxides, it is known that TiO2 (titania) is the material having the highest efficiency. Three phases of TiO2 are known: brookite phase stable at room temperature, anatase phase and rutile phase stable at high temperature. The crystal structure of rutile phase has lower adsorbability to reactants as compared to the crystal structure of anatase phase [J. Phys. Chem., 94, (1990) 8222], shows a low rate of recombination of electrons with holes generated by light, and thus provides low activity as a photocatalyst as compared to anatase [J. Am. Chem. Soc., 103 (1981) 6324; J. Chem., 14 (1990) 265]. Therefore, it is preferred that titania as oxide maintains the crystal phase of anatase in the phtoelectrode of a dye-sensitized solar cell.
A mesoporous titanium dioxide thin film structure using an amphiphilic polymer as a support according to the related art has a large surface area on which a dye can be adsorbed, an interconnected structure that allows effective electron transfer, and a mesoporous structure that helps infiltration of a solid-state electrolyte having a relatively high molecular weight. However, such a mesoporous titanium dioxide thin film structure according to the related art is disadvantageous in that it does not have a thickness of several micrometers suitable for an effective dye-sensitized solar cell and high mechanical strength.
Recently, a so-called layer-by-layer approach and titanium dioxide monolith approach have been studied and reported in the art in order to provide a mesoporous titanium dioxide film with a thickness of several micrometers suitable for a dye-sensitized solar cell (Zukalova, Zukal et al., Nano Lett., Vol. 5, 1789, 2005; Docampo et al., Adv. Func. Mat., Vol. 20, 1787, 2010). However, the mesoporous titanium dioxide films obtained by the above-mentioned methods have a weak connection structure to a conductive substrate, and thus are disadvantageous in that they cannot provide high mechanical strength that is an important factor required for a dye-sensitized solar cell.